Production of isoprene



United States Patent 3,209,048 PRODUCTION OF ISOPRENE Emmett H. Burk, Jr., Hazel Crest, and William D. Holiman, Park Forest, 11]., assignors, by mesue assignments, to Sinclair Research, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed July 6, 1961, Ser. No. 122,082 3 Claims. (Cl. 260-680) This application is a continuation-in-part of now abandoned application Serial No. 94,955, filed January 11, 1961.

This invention relates to a process for the production of isoprene, more particularly the present invention relates to the production of isoprene by the thermal pyrolysis of 3,3-dimethyl-aeolefins.

In view of the similarity of cis-polyisoprene, i.e., synthetic natural rubber, to natural rubber, the demand for isoprene is increasing. The availability and cost of isoprene, however, present formidable barriers to the commercial production of synthetic natural rubber. Thus, there is a continuing search for new sources of isoprene production.

A great deal of research has been directed recently to the production of isoprene by dehydrogenation of C stocks by processes similar to those employed for the production of butadienes. However, these processes have disadvantages which limit their usefulness. Dehydrogenation of isopentane, which is available in relatively large quantities from normal refinery streams and natural gas or field gas streams, is ordinarily carried out by a one step process such as a process using a chromium-alumina catalyst. In this process, isomerization occurs during dehydrogenation resulting in the formation of one pound of piperylene (straight chain C diolefins) for every 2.5 pounds of isoprene in the product.

Furthermore, the quantity of isoamylenes which are available from catalytic cracking or stream cracking of gas oil is quite limited relative to the potential demand for isoprene according to the predicted potential demand for isoprene according to the predicted shortage of 600,000 long tons of natural rubber by 1965. Moreover, none of the catalytic methods is capable of effecting dehydrogenation to isoprene in a desirably selective manner. In all cases, an appreciable amount of the hydrocarbon feed and/or reaction products is consumed in side reactions such as cracking, polymerization and oxidation.

It has also been reported that the thermal cracking of olefins such as Z-methylpentene-Z affords a relatively simple method for isoprene production. In obtaining maximum selective yields under this process, however, it has been found that the rate of cracking is relatively low. Low conversion levels are disadvantageous in that they complicate the separation of the isoprene from the crude pyrolyzate stream.

We have now found that isoprene in high selective yields can be obtained by the thermal cracking or pyrolysis of 3,3-dimethyl-a-olefins and that the process can be run at high conversion levels (e.g.:or about 75%) without formation of n-pentenes thereby greatly simplifying isoprene separation from the crude product stream. We have also discovered that the thermal cracking of the 3,209,048 Patented Sept- 28, 1965 "ice feedstock of the present invention in the presence of HX wherein X is a halogen having an atomic number of 17-53, such as chlorine, bromine, or iodine, results in surprisingly high selective yields of isoprene at high conversion levels with large concentrations of isoprene in the C reaction product. The hydrogen halide may be added as such or it may be formed in the system.

The olefin feedstock of this invention has the formula:

wherein R is an alkyl radical of 1 to 6 carbon atoms, straight or branch chained and with the higher number carbon atom radicals (i.e., of 3 to 6 carbons), the branched structure being preferred. Suitable olefin feeds included, for instance, 3,3:dimethylbutene-1; 3,3-dimethylpentene-l; 3,3-dimethylhexene-1; 3,3,4-trimethylphentene- 1; 3,3-dimethyl-5,S-dimethylhexene-l; etc. The olefins can be substituted with non-interfering groups if desired.

In accordance with the process of the present invention the 3,3-dirnethy1-u-olefin is subjected to thermal cracking generally at a temperature of about 1000 F. to 1500 F. under vacuum or in the presence of an inert gas diluent, such as helium, steam, etc. Operating conditions can be adjusted to give an olefin partial pressure of up to about 0.5 atmosphere, for instance about 0.05 to 0.5 atmosphere, and a contact time suflicient to produce isoprene, for instance, about .00001 to 0.1 second. The inert gas, if employed, can be supplied in a molar ratio of at least about 1 mole, for instance, about 1 to 40 or 100 or more moles, preferably 15 to 30 moles per mole of hydrocarbon. Under preferred conditions the olefin partial pressure is about 0.1 atmosphere and the contact time is about 0.05 to 0.1 second. A reaction temperature of about 1200 to 1400" F. is preferred. If desired, the inert gas can be employed as the heating medium to bring the feedstock rapidly to the cracking temperatures. This can conveniently be done by heating the inert gas to a temperature above that desired for conducting the cracking operation, generally at least 25 C. higher than the reaction temperature and not above say 200 C. of the reaction temperature. The inert gas is then quickly mingled with the hydrocarbon which is at a temperature below that at which any reactions occur. The temperature to which the inert gas is to be heated can be readily determined from the specific heat of the gas, the molar ratios involved, etc. The product from the thermal cracking process is quenched to a temperature of about 500 F. or below and fractionated in ordinary fractionating equipment to obtain isoprene of at least about purity.

The 3,3-dimetbyl-a-olefin feed of the present invention can be prepared by any method known to the art. A feed may be obtained by a process which comprises dimerization of acetone by aluminum amalgum to form pinacol, rearranging the pinacol to pinocolone by treatment with an acid catalyst, reducing the pinocolone by catalytic methods to the corresponding alcohol, 3,3-dimethylbutanol-2, forming a xanthate or other appropriate ester of the alcohol and subjecting it to thermopyrolysis to obtain 3,3-dimethylbutene-1. 3,3-dimethylbutene-1 has been obtained by the pyrolysis of the stearic acid ester of 3,3- dimethylbutanol-2 (see Koch and Van Raay, Greenstoff- Chemie, 32, 161-174, 1951). The preferred, and more practical process for the production of 3,3-dimethylbutone-1 is disclosed in application Serial No. 94,956 to Emmett H. Burk, Jr., and William D. Hoffman, filed January 11, 1961. Briefly the process of the application involves dehydrochlorinating 1-chloro-3,3-dimethylalkane by contact with a solid inert contact material at a temperature of about 450-600 C. and a residence time of about .01 to 10 seconds.

The hydrogen halide additive that may be used in the present invention can be hydrogen chloride, iodide or bromide per se or it can be the iodide, chloride or bromide compounds, which decompose and/or dissociate under the reaction conditions to produce hydrogen halide and whose presence is not otherwise detrimental to the desired reaction. Illustrative of suitable halides are bydrocarbon halides such as methyl halide, tertiary butyl halide, carbon tetrachloride, or inorganic halides such as hydrogen halide. The preferred halide is the iodide. If employed, at least about 1 mole percent of the halide, preferably 2 to 15 mole percent of the halide should be used. There does not appear to be any advantage in employing ever say about mole percent of the halide based 30 on the 3,3-dimethyl-a-o1efin.

The following examples will serve to illustrate the present invention.

EXAMPLE I Table l Table II Experiment N0 1143-181 114318II 1143-18111 Olefin Feed 2,3-limethy1butene-1 Concentration Halide, M01. per- 0 0 Temperature, F. 1,250 1, 350 1, 450 Residence Time, s 0.1 0. 1 0. 1 M01. He/Mol 0lefin-- /1 100/1 100/1 Product Distribution:

Propylene 0.7 2. 9 Isobutylene... 1.0 4. 1 10. 2 Butadiene.-.- 2.0 5. 9 3-methylbutene- 0. 2 Unknown (1).... 0. 8 0.8 2-methy1butene-1 7 2. 5 0. 6 3,3dimethy1butene-1. 2-methy1butene-2..-.- 7. 5 12. 6 6.3 Isoprene 5. 0 20. 8 27. 1 2,3-dimethylbutene-2 2. 7 3. 7 2,3-dimethy1butene-1 74. 2 26. 2 O. 2 2-methylpentene-2 1. 9 5. 6 0. 5 Unknown (2) Unknown (3) 2. 5 G. 1 Gas and Condensation Products. 3. 5 14. 6 45. 4

e gas Mols Isoprene/IOO Mols Feed 23. 9 34. 8 33. 6 Percent Conv. of Olefin 25.8 73. 8 99. 68

EXAMPLE II 2,3-dimethylbutene-1 was thermally cracked under the conditions shown in Table II and analyzed. The results are also reported in Table II.

Examination of the data of Tables I and II clearly indicates that the rate of cracking of 3,3-dimethylbutene-1 is greater than the rates of either of the other two olefins. At 1250 F. 3,3-dimethylbutene-1 was 83.4 percent converted while the other two olefins gave only about 25 percent conversion under the same conditions. At equal conversion levels the yield of isoprene is approximately the same for the three olefins.

Table I also demonstrates the pronounced enhancing eifect the addition of a halide has on the yield of isoprene from the cracking of 3,3-dimethylbutene-l. Comparing runs 1143-17-11 and 1143-17-VI which were run at the same temperature, the yield of isoprene increased from 22.1 moles of isoprene per 100 moles olefin consumed to 71.4 moles when CH I is added. Lower concentrations of methyl iodide give the same type of reaction.

Run 1143- 1143- 1143- 1143- I 1143- 1143- 1143- 17-III 17.II 17-1 17-1V 17-VI 26 16-1 Olefin Feed 3,3-dimethylbutene-1 Z-methylpentene-2 Concentration:

Halide, M01 percent 0 0 0 1 20 2 5 Temp., F 1, 250 1,340 1, 445 1, 490 1, 340 1, 250 1, 250 Residence Time (sec 0.1 0.1 0. 1 0 1 0. 1 0. 0.1 M01 He/Mol Olefin lOO/l 100/1 100/1 100/1 100/1 100/1 100/1 Product Distribution, Wt. percent of Feed:

Proyplene 0.3 0.6 1. 1 2. 5 0.3 Isobutylene. 6. 9 7. 4 8. 9 8. 5 2. 9 Butadiene 2.0 3. 0 4. 4 6. 9 0.4 B-methylbutene- 0. 1 Unknown 0.6 0.9 1.0 0.2 0.1 2-methy1buteue 1. 1 1. 3 0.7 0.2 1. 1 3,3-dimethylbute 14. 6 0. 7 13. 3 2-methylbutene-2. 9. 3 3. 7 3. 4 0. 9 3. 4 Isoprene 20. 4 17. 8 20. 5 11. 2 50. 1 2,3-dimethylbutene- 2,3-dimethylbutene-1 1. 1 0.8 0. 9 0. 2 0. 2 2-methylpentene-2 8.0 1.8 0.6 0.3 1 6 75. 6

nknown 2. 6 0.7 Unknow 1. 9 Gas and Condensation Products.. 31. 2 61.0 58. 6 67. 6 27. 7 39.1 3 5. 4 Mols Isoprene/IOO M01 Feed Consumed 30. 2 22.1 25. 4 13. 8 71. 4 49. 5 44.1

Percent C0uv., Olefin 83. 4 59. 6 100.0 100.0 86. 7 100. 0 24. 4

1 M01 Percent CHzI. 2 M01 Percent n-Butyl Br. 3 Gas EXAMPLE III Example VI The pyrolysis of 3,3,4-trimethylpentene-l was carried out in the same equipment and in the same manner as Example V. Hydrogen iodide was employed as the hal- III. The results are also shown in Table III. 5 o dat f thi run is shown in Table VI.

Table III Temperature, -1,350 F., 0.047 sec. Temperature, -1,440 F., 0.047 sec.

Contact Time Contact Time Run 1148-850 1192414 1148-9513 1192-70 1148-87B 1192-3B 1148-95B 1192-714 Conversion 55. 7 59. 3 57. 9 58. 9 95. 2 92. 3 92. 4 95. 5 Molar Yield of Isoprene 25. 6 52. 6 41. 2 34. 4 33. 6 56. 41. 39. 4 Weight Percent of Additive None 1 2. 8 2 10.0 3 10.8 None 1 2 10 3 10.8 M01 steam/M01 olefin 80/1 80/1 80/1 80/1 80/1 80/1 80/1 80/1 Composition of C5 Hydrocarbons, Wt. Percent:

2-MeC4= 11. 2 5. 9 7. 2 8. 7 6. 8 4. 4. 4. 6 4. 5 33. 6 22. 4 24.8 23. 6 20. 3 10. 3 13. 9 12. 0 55. 2 71. 7 68. 0 67. 7 72. 9 84. 0 81. 5 83. 5

1 Wt. Percent CH3I. 2 Wt. Percent t-Butyl Br. 3 Wt. Percent C014.

EXAMPLE IV Table VI 3,3-dimethylbutene-1 was thermally cracked in the R un 1211 39 presence of various concentrations of iodide as in Exam- Conditions, ple I under the conditions shown in Table IV. The reo sults are also shown in Table IV Temperature n 1127 H O, moles/hr. 9.94 Table IV H'C, moles/hr. .405 [Temp. 1,400 F., .047 sec. contact time] mole Percent (based on Pressure (mm.) 780 1192-413 114893B 1148-948. Converslon, Percent 84 Products, ultlmate wt. percent: 6./4 grt z 2 80 1 011.1 t-But t-But I 17 8 Molar Yield of Isop 53.1 60. 4 62. 1 3 gonversign..qfififfihhfihfl- 78. 8 8 7 C4: 0.5

omposr 1on0 y ocar ons bg'lififi f 4 7 3 8 5 5 40 Zfi g j n e 4= 6 2-MeC =2 21.6 17.8 16.8

Isoprene 73. 7 78. 4 77. 7 2MeC4=2 5 Isoprene 35.00 EXAMPLE V Hlgher bolllng products 0.78 Molar selectlvrty to lsoprene 57.8 A 6-inch furnace was equipped w1th a quartz reactor,

utilizing powdered quartz as a fluidized heat transfer medium. 3,3-dimethylpentene-1 was introduced into a stream of steam at a temperature of 500 F. The mixture was passed through the reaction zone under the conditions in Table V below. The products from the cracking zone were collected, weighed and analyzed. The results are shown in Table V, wherein one run was carried out in the absence of halogen and the other with hydrogen iodide.

Table V Run 1211- 10 12 Conditions:

Temperature, T... 1, 224-1, 242 1, 211-1, 242 H2O, in es r- 10 10 HC, moles/hr 0.29 0.29 HI, mole percent (based on HC) 0. 0 4. 8 Pressure (mm. 800 780 Conversion, percent 50. 5 65. 5 Products, Ultimate Wt., Percent 0.17 0.06 3.14 4.80 15. 9. 44 1. 98 12.90 1. 26 0. 17 0. 43 5.09 2.04 0.69 0. 29 0.50 1. 88 2.02 3. 32 18. 60 15. 60 23. 80 45.00 Higher Boiling Products. 26. 10 3. 71 Coke 0.12 0. 09 Molar Selectivity to Isoprene 34. 3 64. 9

We claim:

1. A process for producing isoprene which comprises subjecting olefin to thermal pyrolysis at a temperature of about 1000 to 1500 R, an olefin partial pressure of up to about 0.5 atmosphere, and in the presence of at least about 1 mole percent of hydrogen iodide, said olefin being of the formula:

CH=CH wherein R is an alkyl radical of 1 to 6 carbon atoms.

2. A process for producing isoprene which comprises subjecting 3,3--dimethylbutene-1 to thermal pyrolysis at a temperature of about 1000 to 1500 F., a 3,3-dimethylbutene-l partial pressure of up to about 0.5 atmosphere and in the presence of an inert gas in a molar ratio of at least about 1 mole of inert gas per mole of 3,3-dimethylbutene-l and at least about 1 mole percent of HI.

3. A process for producing isoprene which comprises subjecting 3,3-dimethylbutene-1 to thermal pyrolysis at a temperature of about 1000 to 1500 F., a 3,3-dimethylbutene-l partial pressure of up to about 0.5 atmosphere and in the presence of at least about 1 mole percent of hydrogen iodide.

(References on following page) 7 8 References Cited by the Examiner 1,222,736 1/ 60 France.

841,351 7/60 Great Britain. UNITED STATES PATENTS 868,566 5/61 Great Britain. 2,397,638 4/46 Bell et a1. 260-683 3,115,531 12/63 ColtOIl et 260680 5 PAUL M. COUGHLAN, Primary Examiner.

FOREIGN PATENTS ALPHONSO D. SULLIVAN, Examiner.

588,870 7/60 Belgium. 

1. A PROCESS FOR PRODUCING ISOPRENE WHICH COMPRISES SUBJECTING OLEFIN TO THERMAL PYROLYSIS AT A TEMPERATURE OF ABOUT 1000 TO 1500*F., AN OLEFIN PARTIAL PRESSURE OF AT LEAST ABOUT 1 MOLE PERCENT OF HYDROGEN IODIDE, SAID OLEFIN BEING OF THE FORMULA: 